Kirkendall effect-induced uniform stress distribution stabilizes nickel-rich layered oxide cathodes

Nickel-rich layered oxide cathodes promise ultrahigh energy density but is plagued by the mechanical failure of the secondary particle upon (de)lithiation. Existing approaches for alleviating the structural degradation could retard pulverization, yet fail to tune the stress distribution and root out the formation of cracks. Herein, we report a unique strategy to uniformize the stress distribution in secondary particle via Kirkendall effect to stabilize the core region during electrochemical cycling. Exotic metal/metalloid oxides (such as Al2O3 or SiO2) is introduced as the heterogeneous nucleation seeds for the preferential growth of the precursor. The calcination treatment afterwards generates a dopant-rich interior structure with central Kirkendall void, due to the different diffusivity between the exotic element and nickel atom. The resulting cathode material exhibits superior structural and electrochemical reversibility, thus contributing to a high specific energy density (based on cathode) of 660 Wh kg−1 after 500 cycles with a retention rate of 86%. This study suggests that uniformizing stress distribution represents a promising pathway to tackle the structural instability facing nickel-rich layered oxide cathodes.

the heterogeneous nucleation process.The bonding energy across the substrate interface was compared in Fig. S2, 1  From the perspective of the nucleation energy.The co-precipitation reaction involves the nucleation of primary grains and the growth of precursors, wherein the nucleation reaction was gradually completed after the diffusion process of transition metal ions in the salt solution from pH 5.4 to 11 2 .The nucleation work of homogeneous nucleation was calculated as follow: Wherein γ represents the surface free energy or surface tension; ΔGV represents ΔG on a per unit volume basis.∆ * represents the energy barrier for nucleation.
As for heterogenous nucleation, nucleation processes are catalyzed by a heterogeneity such as an accommodating substrate surface.
The energy barriers for heterogeneous nucleation are much lower than those of homogeneous nucleation.In addition, heterogeneous nucleation is widely recognized to feature higher nucleation rate and finer grain size 3 .Thus, the nickel hydroxide precursors grown on the Al2O3 exhibited a dense packed microparticles with distinct core-shell structure.As a result, aluminum oxide which was not covered by precursor material is not observable on the early stage of co-precipitation reaction.
shell structure with Al2O3 seeds located in the core.During the annealing process, the Al atoms will be gradually migrated to the outer surface leaving a gradient doping of Al to the secondary particle with higher concentration of Al at the core region and lower concentration at the outer surface.According to the Arrhenius equation 4 , there is an exponential relationship between the diffusion coefficient (D) and temperature, which implies that the thermal motion of the atoms intensifies and the diffusion coefficient rises sharply as temperatures increase.
where R is the gas constant and Q is the activation energy.D0 is the diffusion constant.Temperature has a strong impact on the diffusion constant.In previous studies, oxide powders have been widely used as dopants. 5,6Higher Al doping will result in smaller primary particle size than those with lower Al doping 7,8 , as verified by Fig. S7ej.

Supplementary Figures
Fig. S1 SEM image of Al2O3 seeds.Scale bar, 1 μm.DSC profiles of the two samples (Fig. S17a) show the higher thermal stability of hk-LiNi0.96Al0.04O2.The c-LiNi0.96Al0.04O2,which has the same Ni content (96%) as that of hk-LiNi0.96Al0.04O2,delivers a notable peak temperature at 215 °C.For hk-LiNi0.96Al0.04O2, the peak appears at 224 °C, and the enthalpy quantification of the exothermic peak is 1743.37Jg −1 , which is much lower than that of c-LiNi0.96Al0.04O2 (2412.85 J g −1 ).This implies less oxygen release during the structure collapse near the material surface.During the heating, the layer-structured hk-LiNi0.96Al0.04O2 completely transform into a spinel structure at 250 °C (Fig. S17b, c), higher than c- including Al2O3 to Al2O3, xNi(OH)2 to (1-x)Al2O3 and Ni(OH)2 to Ni(OH)2 as shown.The self-bonding energies of Al2O3-Al2O3 and Ni(OH)2-Ni(OH)2 were 0 and 5.3 kJ/mol, respectively.In this work, the molar ratio of Ni to Al was accurately preset at 96:4, and the bonding energy of the corresponding composition points (corresponding to red star in Fig. S2) should be larger than those self-bonding energies.When Al2O3 oxide seeds are added into the reaction solution, due to the fact that the bonding energy of xNi(OH)2-(1-x)Al2O3 is stronger than that of Ni(OH)2-Ni(OH)2 (Fig. S2), These energy values suggest that on one hand alumina would not agglomerate into larger particle, and on the other hand the Ni(OH)2 precursors has a stronger tendency to grow on the alumina rather than forming the individual Ni(OH)2 particles when Al2O3 oxide seeds are added into the reaction solution.This can be verified by the SEM images shown in Fig. S4.The growth process of Ni(OH)2 layer on alumina is prior to the spontaneous aggregation of Ni(OH)2 grains.

Fig. S2
Fig. S2 Bonding energy of Al2O3 and Ni(OH)2 as a function of component percentage of Ni(OH)2 in the mixture.

Fig
Fig. S3 a SEM images of single Al2O3 seed at pristine and b during heterogenous nucleation of Ni(OH)2 grains.Scale bar, 500 nm.

Fig
Fig. S5 Distribution of Al and Ni within precursors.a Cross-sectional SEM images and mapping photograph of Ni (b), Al (c) and O (d) within precursor particles for hk-LiNi0.96Al0.04O2.Scale bar, 20 μm.

Fig
Fig. S7 Internal morphology within the secondary particle.SEM images of (a, c) c-LiNi0.96Al0.04O2and (b, d) hk-LiNi0.96Al0.04O2secondary particles with a close particle size distribution and different primary particle morphology.Cross-sectional images and the corresponding schematic illustration of the c-LiNi0.96Al0.04O2(e, f) and hk-LiNi0.96Al0.04O2(h, i).g, j Primary particle size distribution of the two samples, as a function of the distance from the center of secondary particle.Scale bar, 30 μm a, c; 3 μm b, d, f, i; 1 μm e, f, h, i.

Fig. S8
Fig. S8 Volume change and SOC heterogeneity within secondary particle.a Scheme of the resulting secondary particle with interior-rich Al dopant and central void structure.b Volume change of micro-lattice as a function of charge voltage under different aluminum doping amounts.c The evolution of the lattice parameter c as a function of delithiation for the two separated phase.d Two-phase separation within hk-LiNi0.96Al0.04O2compared with the control sample during H2-H3 phase transition.

Fig. S9
Fig. S9 The nano-indentation experiment.a, b SEM image captured the nanoindentation approach before and after mechanical failure of the representative c-LiNi0.96Al0.04O2and hk-LiNi0.96Al0.04O2particle.c Corresponding force-displacement curves for indentation phases of the two samples.Scale bar, 5 μm a, b.
LiNi0.96Al0.04O2(220°C), as evidenced by merging of the (108)R and (110)R peaks into a single peak attributed to spinel (440)S.Then both the two disordered spinel phase samples start to transform into rock salt structure (Fm3̅ m) at 320 °C.Less interfacial exposure in the charging state could explain these observations in the thermal stability test.

Fig. S18 XPS
Fig. S18 XPS spectra of cycled samples.a-c, XPS spectra of C 1s (a), O 1s (b) and F 1s (c) for electrodes retrieved from half cells (vs Li/Li + ) were required without sputtering.The cells were cycled for 300 times between 2.8 and 4.4 V using a constant current of C/3 (about 60 mAg −1 ) at 35 °C.d Chemical composition of the two samples surface.Atomic percent on the samples surface were obtained from XPS spectra.

Figure S22 a
Figure S22 a Cross-sectional SEM image of the precursor for hk-LiNi0.8Co0.15Al0.05O2cathode materials obtained by the developed co-precipitation method and b the corresponding element mapping of the Al2O3 seed within single hk-LiNi0.8Co0.15Al0.05O2precursor.c Cross-sectional SEM image of the precursor for c-LiNi0.8Co0.15Al0.05O2by classical co-precipitation method under the same synthesis conditions.Scale bar, 1 μm.